Data networks for transferring electronic information are becoming increasingly widespread for the communication of many different types of data including text, graphics, voice, and video data used with a computer. Such networks enable the interconnection of large numbers of computer workstations, telephone and television systems, video teleconferencing systems and other facilities over common data links or carriers. Computer systems are typically interconnected by local area networks (LAN) such as Ethernet, Token Ring, DECNet, and RS-232 networks. Metropolitan, national and international network systems for computers are interconnected by wide area networks (WAN) such as T1, V3.5 and FDDI.
With the advent of LANs and WANs, use of networks of various types to connect computer systems continues to increase. As the network is required to communicate a significant amount of information, performance of the network is often evaluated and many techniques have been developed and used to enhance data flow within such networks.
The precise data transmission techniques used within computer system networks depends on both the hardware and systems architecture of the network. The International Organization for Standardization (ISO) has promulgated an Open Systems Interconnection (OSI) communications model for use with data communications between computer systems and has been incorporated in many network architectures. The OSI model has several different layers which include a physical layer, a data link layer, a network layer, a transport layer, a session layer, a presentation layer and an application layer. Other structured architectures, such as a Systems Network Architecture (SNA), have a similar organization, although the various layers may not be exact equivalents of the OSI model.
Protocols between communicating computer systems are often implemented at multiple layers of the structured model. For example, the physical layer uses various signalling protocols, and the data link layer insures that individual data packets are not corrupted during transmission between two directly connected systems. At the same time, the network and transport layers insure that data packets arrive at the correct systems within a network and in a correct order. Higher layers also talk to one another using various preselected protocols. Such protocols are well-known in the data processing art and will not be described in greater detail herein. Additionally, there is also class of protocols which do not correct corrupted packets and do not ensure that the data packets arrive in a correct order. Such protocols may include a DataGram service, referred to as UDP/IP (User Datagram Protocol/Internet Protocol).
A technique which has been used to control data flow over networks at the session level is referred to as session level pacing. This technique has been used in structured architectures, such as systems network architecture (SNA). With this technique, a receiving logical unit controls when a sending logical unit may send a window of data packets. It should be noted that a "window" represents the total number of data values in transit through the network at any time. This methodology utilizes explicit acknowledgment signals sent from a receiving logical unit to indicate that a window of data was received without error. This acknowledgment signal is then used to advance the window to admit a new set of data values. In the static pacing technique described above, the window size is fixed when the communication session between sending and receiving logical units is initialized. Only the timing of the window transmission is controllable in this static pacing technique.
An adaptive pacing technique has also been used to allow a window size to be adjusted dynamically during the communication session. In an adaptive pacing technique, the receiving logical unit controls a size of each window in order to optimize use of its own resources. This adaptive pacing technique is described in detail in U.S. Pat. No. 4,736,369, issued to Barzilai, et al., which is hereby incorporated by reference herein.
Although adaptive pacing allows more efficient use of both the communications link and receiving logical unit resources, problems may arise at the sending logical unit. For example, even the adaptive pacing technique requires that the sending logic unit keep a free window of data available until the receiving logic unit transmits an acknowledgment for the window and the sending logic unit receives the acknowledgment. Therefore, while the sending logic unit is waiting to receive affirmation that the receiving logic unit has in fact received the window of data, the sending logic unit is unable to send any additional data and valuable bandwidth is wasted. Additionally, because the sending logic unit is required to wait a certain number of timing cycles until an acknowledgment is received, real-time transfers of data are not easily implemented. When such real-time operations are transmitted, a resulting display at the receiving logic unit fails to provide a true transmission of the desired information.
Therefore, it is desirable to implement a system which transfers data to communicate information in a network in a real-time manner.